1. Field of the Invention
The present invention relates to a communication system, a communication apparatus and method and a program, and more particularly to a communication system, a communication apparatus and method and a program capable of suppressing a communication process speed from being lowered by signal collision.
2. Description of Related Art
In a conventional communication system, as signals are transmitted from a plurality of transmission apparatus at the same time to the same communication medium, a signal collision occurs so that a reception apparatus cannot receive signals correctly in some cases. Particularly in the case wherein devices respond to a request transmitted from one device through broadcasting, there is a high possibility that response timings of the devices become coincident with each other, and a probability of signal collision occurrence becomes high.
For example, in a non-contact type IC card system for short distance wireless communications, when a reader/writer requests identification information from IC cards existing in a communicable range, the reader/writer transmits the request through broadcasting (transmits the request to many and unspecified IC cards). If there exist a plurality of IC cards in the range communicable with the reader/writer, each IC card responds to the request and transmits its identification information. However, since a possibility of coincident transmission timings becomes high, there is a high fear that the reader/writer cannot acquire correctly each identification information piece, because of signal collision occurrence.
In this connection, for example, there is a method of avoiding a signal collision by assigning a specific code and frequency to each device. However, if the number of devices (in this case, IC cards and a reader/writer) is large such as in a non-contact type IC card system for short distance wireless communications, codes and frequencies to be assigned become insufficient.
In this connection, there is a method of intentionally shifting timings of signal transmissions from devices by providing a communication process with time slots (for example, refer to Japanese Patent Application Publication No. 2003-317042). Namely, one signal transmission process is provided with a plurality of temporal transmission timings (time slots). When a signal is transmitted, each device generates a random number, and transmits the signal at the timing (time slot) corresponding to the generated random number. In this manner, it becomes possible to suppress a signal collision occurrence, because signal transmission timings are dispersed.